Acid gases in refineries, substantially comprising H2S and CO2, mainly come from acid water strippers, recycled hydrogen desulfurization apparatuses, dry gas desulfurization apparatuses, and the like. Currently, acid gases in most small refineries are basically discharged after being combusted. This method, on the one hand, is a waste of resources, and on the other hand, brings about tremendous pressure to environmental protection and affects business development. In order to protect the environment and ensure sufficient utilization of resources, acid gas recycling in small refineries is rather imperative.
In medium and large-sized refineries, acid gasses are mainly used for preparing sulfur, currently, through two processes, i.e. one involves the secondary Claus, exhaust hydrogenation, plus solvent absorption process, while the other adopts the LO-CAT process developed by Merichem Company which focuses on gas technology products.
The secondary Claus, exhaust hydrogenation, plus solvent absorption process is featured by process maturity, stable operation, and stable product quality of sulfur. However, due to long process procedures and large investment, the Claus process can only be used for treatment of high concentrated acid gasses. Operation difficulties might arise when the volume fraction of H2S in the gases is lower than 20%. As a result, the Claus process is suitable for equipment having an annual output of sulfur more than 5,000 t.
The LO-CAT process uses a multi-chelated iron catalyst to directly convert H2S into the element of sulfur, with a removal rate of H2S over 99.9%. The LO-CAT process can be adapted to various operating conditions, such as high amount fluctuations of acid gasses and concentration fluctuations of H2S in the range from 0% to 100%. Hence, the LO-CAT process can be practically used to process various sources in different conditions. Besides, the LO-CAT liquid redox process does not use any toxic chemicals, nor will it produce any harmful by-products. Moreover, environmentally safe catalysts can be continuously regenerated in the process. However, the LO-CAT process is subject to high operation costs, inferior purity and color of sulfur to those achievable in the Claus process, and occurrence of blockage due to sulfur particles generated in the production process. Therefore, the LO-CAT process is accompanied with lower economy efficiency in equipment with an annual sulfur production below 5,000 t (than the secondary Claus, exhaust hydrogenation, plus solvent absorption process can achieve).
Since small refineries produce relatively small amounts of acid gasses, using the secondary Claus, exhaust hydrogenation, plus solvent absorption process therein results in the problems of long process procedures, complicated operations, large investments, and poor scale benefits. And use of the LO-CAT process in small refineries also leads to the problems of large one-time investments, high costs of catalysts and patent licensing fees, and the like. Therefore, small refineries which produce small amounts of acid gases can employ a new desulfurization process which requires smaller investments to recover H2S and prepare sulfites. In the new process, acid gasses are first burned to generate SO2, which is then fed into an adsorption column for chemical absorption to generate a sulfate solution. The solution is subsequently reacted with an alkaline absorbent to produce liquid or crystal products of sulfites, form which solid sulfite products can be prepared through separation, drying, and other procedures. The new process is characterized by short procedures, simple reactions, and flexible operations, and can be adapted to the influences imposed upon the production process by fluctuations of acid gasses in small refineries. The solid or liquid products can be produced by selecting different procedures, and different absorbers can produce different types of sulfites. Exhaust emission standards are achievable through three-stage absorption, so as to achieve the purpose of purifying exhaust gasses. In actual production processes, however, there exists problems of serious equipment corrosion and high maintenance costs.
CN 101143714A discloses a method for preparing sulfuric acid by an acid gas with a high concentration of hydrocarbons. The acid gas hydrogen sulfide enters a first hydrogen sulfide burning furnace and a second hydrogen sulfide burning furnace respectively in proportion for combustion. High temperature furnace gas coming out from the first burning furnace is first cooled down to a certain temperature by air through a furnace gas cooler, and then enters the second burning furnace to burn with a supplementary acid gas containing hydrogen sulfide and residual air in the furnace gas. High temperature furnace gas coming out from the second burning furnace enters a waste heat boiler for heat accumulation, and then enters a purification section, a conversion section, and a dry absorption section for conventional acid production. This method is only capable of producing 98% of industrial sulfuric acid but fails to produce fuming sulfuric acid which is of higher values. Meanwhile, difficulties in transportation and storage of the sulfuric acid render stable market requirements in nearby refineries critical to the development thereof.
CN 1836767A discloses a method for treating acid gases from oil refineries. The acid gases are used as a fuel in the vertical kilns of a cement plant. During the combustion of the acid gases in the kiln, the component of H2S thereof is reacted with cement materials to produce CaSO4, while other harmful components are sintered and converted as well. This method fundamentally solves the problem of acid gas treatment while providing fuels to cement plants, reaching dual purpose of protecting environment and providing fuels. However, this method is also somewhat limited and difficult to be promoted.
CN 101337661A discloses a method for preparing sodium hydrosulfide. Caustic soda and lime cream are used to absorb an acid gas containing H2S and CO2 to generate intermediate solutions respectively, and then the above two intermediate solutions are mixed at a certain ratio to obtain a sodium hydrosulfide product having a low content of carbonate ions. This method does not require a high concentration of H2S in the acid gas, but the procedures thereof are rather long and are of a low degree of automation.
In “Industrial technology of preparing sodium sulfide from absorption of hydrogen sulfide with sodium hydroxide solution” (Shang Fangyu, Inorganic Chemicals Industry, vol. 44(2), February 2012), it discloses a process of absorbing hydrogen sulfide by a sodium hydroxide solution to prepare sodium sulfide, wherein a sodium hydroxide solution at a concentration in the range from 380 g/L to 420 g/L is used for absorbing hydrogen sulfide in a packed column. The mass concentration of sodium sulfide at the end of the reaction is controlled within the range from 330 g/L to 350 g/L, with an absorption rate of hydrogen sulfide in the range from 95% to 98%. This process does not only provide effective way of environment protection, but also produce benefits for companies. However, the sodium sulfide in the process is easily subject to deterioration and difficult to be stored.
Based on the foregoing, small fineries are now in urgent need of a comprehensively good (safe, environmentally friendly, economic, etc.) process for processing acid gasses.